Polynuclear Complexes of Nd and Dy with N₂O₃ Donor Ligands: Solution Speciation and Selective Precipitation Studies

This study explores the formation and selective separation of polynuclear neodymium and dysprosium complexes with N₂O₃ donor ligands derived from N,N-bis(salicylidene)-1,3-diamino-2-propanol (H₃L^H). The research focuses on the structural characterization and solution speciation of Nd³⁺ and Dy³⁺ complexes by using ligands with different peripheral substituents: H₃L^H, H₃L^p-OMe, and H₃L^o-tBu. These substituents significantly influence the nuclearity of the resulting complexes. For Dy³⁺, single-crystal X-ray diffraction (SC-XRD) revealed a range of molecular architectures, from dinuclear to hexanuclear species, each with distinct solubility profiles, whereas for Nd³⁺ an heptanuclear molecular structure with H₃L^o-tBu was obtained. Separation experiments with Nd:Dy ratios of 1:1 and 4:1 demonstrated the ability of the ligands to give a partial selective precipitation of Nd³⁺ and Dy³⁺ complexes, depending on both metal identity and ligand structure. In particular, H₃L^H achieved a separation factor (S_Nd/Dy) of 12.0 (±2.0), concentrating Nd³⁺ in the solid phase. In contrast, H₃L^o-tBu favored Dy precipitation, yielding a separation factor of 20.0 (±4) after just 10 min. In various instances, the low separation factor values were ascribed to the formation of mixed-metal polynuclear species, which was confirmed through Electrospray Ionization Mass Spectrometry (ESI-MS) and by the structural characterization of a heteronuclear complex with H₃L^o-tBu.